Change new List(n) to return fixed length list.

runtime/lib/expando_patch.dart

Index: runtime/lib/expando_patch.dart
diff --git a/runtime/lib/expando_patch.dart b/runtime/lib/expando_patch.dart
index 75369a260abf04f9856a63ac5ae2f06a85b2cfe6..bb3e32490f9826c3693c3a6894beba720ba01f5d 100644
--- a/runtime/lib/expando_patch.dart
+++ b/runtime/lib/expando_patch.dart
@@ -21,7 +21,7 @@ patch class Expando<T> {
       }
     }
     if (doCompact) {
-      _data = _data.where((e) => (e != null)).toList();
+      _data = _data.where((e) => (e != null)).toList(growable: true);
     }
     return result;
   }
@@ -49,7 +49,7 @@ patch class Expando<T> {
       _data.add(new _WeakProperty(object, value));
     }
     if (doCompact) {
-      _data = _data.where((e) => (e != null)).toList();
+      _data = _data.where((e) => (e != null)).toList(growable: true);
     }
   }